Various types of switches are used in electronic test and measurement systems to switch or route signals between a stimulus, measuring instruments, and devices-under-test (DUTs). Often the switches that switch the fastest at the lowest currents and voltages have physical contacts that are susceptible to damage. Damage, or abuse, can be the result of electrical transient effects that occur during the make or break cycle of the switch. Measurement can be defined as the process of testing or exercising a circuit using both sources and detectors.
Switches common in the art are reed relays, armature relays, and electronic or solid-state switches.
Reed relays are used in switching platforms that employ thousands of such switches. Replacing thousands of relays at regular intervals or replacing selected worn out relays is an undesirable expense.
While these switch contacts can be protected by the use of current-limiting resistors in the path of the switch, the resistors sometimes add undesired impedance to a circuit.
A majority of the abuse happens to the switch closed last and the switch opened first for a given circuit. The abuse mechanism is different in each case. The mechanism on closure results from a voltage differential across the open contacts. As the switch closes, very high currents can flow from circuit capacitances damaging the switch. Damage on switch closure is due primarily to an excessive surge in current, also called in-rush current. This occurs when the voltage across the contacts equalizes while the instrument and channel capacitances charge or discharge. Small micro-welds develop on the surface of the switch contacts over time and the contacts become irregular and pitted. Subsequently, the contacts develop a higher resistance. Catastrophic failure during closure for most switches is a stuck close condition.
Damage on opening the switch is due primarily to an excessive surge in voltage causing arcing. Current flowing through the inductance of a closed circuit path cannot change instantaneously. As the switch opens, an arc forms to dissipate the energy stored in the inductance. This arc also damages the switch contacts. This arc causes contact pitting in relays as the contacts separate due to excessive power and heat in a very small area. Contact resistance for relays will typically continue to increase as they are repeatedly subjected to arcing. There is no typical failure mode for solid-state switches subjected to an arc, but it can be catastrophic.
One method limiting the in-rush current is to place series resistance in the switch circuit. However many applications cannot accommodate fixed resistance in the path of the switch however. Some switch cards provide both current limited and non-current limited channels while other cards provide methods defeating the current limit on selected channels by installing shorts across the current-limit resistors. Other methods of minimizing in-rush current include minimizing the voltage difference or reducing the capacitance across the open switch contacts, neither of which may be practical. Increasing the series resistance of the path decreases the peak current. This decrease in current lessens the energy stored by the inductance by the square of the current and minimizes the damage. Techniques for minimizing arcing include minimizing the circuit inductance and adding “snubber circuits” across the contacts. Snubbers are circuits that provide a transient path for the current flow immediately after the circuit is opened. Snubbers are common for switching involving high current and/or large inductance, but they are not a universal solution. They place a large residual capacitance across the switch contacts that results in in-rush current on closure and a frequency-dependent leakage path when the switch is open.
Configuring switch cards to form a larger switching structure is desirable when designing large systems using modular blocks. Merely adding additional switching structures in parallel can cause a problem with the additional loading the switches represent. Not all switch cards provide a feature to connect and disconnect a switch card (channel switching structure) to or from the electrical system. In some cases, this option is not desirable as it doubles the number of switch contacts in series with the measuring instruments and the amount of cabling and interconnect required to add it often makes it prohibitive.
Accordingly, a need exists to prevent damage to switches without permanently adding resistance to the measurement path, and providing a feature to easily disconnect a switch card from the electrical system.